Template constraints should introduce identifiers inside their scopes

[Timon Gehr]

On 21.09.22 14:32, Paul Backus wrote:
>>
>> They are not semantically the same. The first can be instantiated with 
>> any type and conditionally makes an output (it is empty otherwise), 
>> the other says it cannot be instantiated unless the arguments have 
>> specific properties.
> 
> If you want the semantics to match exactly you can replace the `static 
> if` with a `static assert` (or add `else static assert(0);`).

I think this is not true.

```d
module a;

template foo(T){
     static assert(is(T==int));
}

template bar(T)if(is(T==int)){}
```

```d
module b;

template foo(T)if(is(T==float)){}
template bar(T)if(is(T==float)){}
```

```d
module c;
import a,b;

alias foo1=foo!int; // ok
alias foo2=foo!float; // error
alias bar1=bar!int; // ok
alias bar2=bar!float; // ok
```

`static assert` can give you a custom error message, but template 
constraints have much better overloading behavior. The two features are 
not comparable. Ideally template constraints would be improved so they 
are a strictly better choice than static assert whenever they are 
appropriate.